E422
Navien Tankless Water Heater
Severity: ModerateWhat it means
Navien E422 is a flow-sensor fault — the little turbine wheel that measures how much water is passing through is reporting flow that's too high or a reading that doesn't make sense.
The usual cause is grit or scale fouling the flow sensor's turbine so it spins wrong, or a recirculation pump set up so the unit sees flow it shouldn't.
It can also be a failed sensor or a wiring fault.
The unit may run oddly or refuse to fire until it's resolved.
Affected Models
- Navien NPE-180A, NPE-210A, NPE-240A (Classic)
- Navien NPE-180S, NPE-210S, NPE-240S
- Navien NPE-2 series
- Navien NCB-E combi-boilers
- Navien NPN series
Common Causes
- Debris, sand, or scale fouling the flow-sensor turbine wheel
- Cold-water inlet filter screen torn or missing, letting grit reach the sensor
- Recirculation pump or crossover valve plumbed/configured wrong, so the unit reads phantom flow
- Flow sensor turbine worn or stuck
- Loose or corroded connector on the flow sensor
- Control board input fault (rare)
How to Fix It
-
Reset and watch.
Power off at the panel, ten seconds, back on.
Run a hot tap and see if E422 returns.
A one-time hit after plumbing work (air or debris stirred up) may not come back.
If it does, the sensor needs cleaning or the recirc setup needs checking. -
Clean the cold-water inlet filter.
Shut the cold isolation valve, unscrew the inlet filter, and check the screen.
If it's clogged, rinse it; if it's torn, replace it — a damaged screen lets grit through to the flow sensor, which is what fouls the turbine in the first place. -
Check your recirculation setup, if you have one.
Navien units with built-in recirc, or an external pump and a crossover valve under a far sink, have to be configured correctly in the unit's settings.
A miswired pump or a crossover valve stuck open makes the unit see continuous flow.
Confirm the recirc mode setting matches your actual plumbing. -
Clean or replace the flow sensor.
Breaker off, water off.
The flow sensor sits on the cold-water inlet side inside the unit.
It can be removed and the turbine wheel cleared of grit and scale; if the wheel is worn or won't spin freely, replace the sensor.
This is doable for a confident DIYer but it's also a routine Navien service job. -
Still faulting? Get it diagnosed.
Filter good, recirc correct, sensor cleaned or replaced — and E422 persists? Then it's the wiring to the sensor or the board input.
That's a service-tech call.